High Dynamic Range Displays

By Jeff Sauer

The human eye is an incredible instrument. It has the ability to process a very high range of colors and even a larger range of luminance variations. Indeed, when we hear display contrast ratios like 500:1, 2000:1, or even 50,000:1, they ultimately pale to the roughly 1,000,000:1 luminance range of our eyes. Similarly, the 24-bit color processing (or 32-bit, including alpha channel) that seemed pretty good a decade ago now feels unsatisfying in the quest to produce lifelike images and video.

And that is just why a small but growing number of applications, some as common as Adobe Photoshop CS2, and computer games are moving toward high dynamic range imagery (HDRI) and creating HDRI with as much as 16 bits per color.

Of course, while the software can process more data, you probably don’t have a monitor that can show it. Naturally, “quality in” begets “quality out,” and maintaining a high dynamic range throughout the creation and editing processes will reduce rounding errors and yield a better image even on a low dynamic range monitor.

Still, a monitor that could display the full quality of imagery would be so much better.

Brightside Technologies recently introduced the first HDR display, its DR37-P, which uses a different backlight compared to typical LCD monitors. While the technology is highly desirable, the monitor’s price, $49,000, may prove to be too steep for the average user.

Now there is at least one such monitor. Brightside Technologies is a small Western Canadian company with the first HDR-capable display. Unfortunately for most of us, Brightside’s high dynamic range display-priced at $49,000-isn’t likely to show up on our individual desktops very soon. But, the technology is eye-opening and, hopefully, it’s a glimpse at the future.

Lighting It Up, and Down

On the surface, Brightside’s DR37-P uses a fairly straightforward LCD panel, similar to any other higher quality 37-inch LCD TV/monitor. But there are a couple of critical variations: First, Brightside uses a much different backlight; and second, that backlight doesn’t just turn on and off with the power switch.

LCD monitors typically create brightness by shining a large, defused backlight through a matrix of liquid crystals. Electric charges cause the liquid crystals to “turn out” to allow some or all of the light to pass through, thus creating different levels of brightness and varying shades of gray. The light then passes through red, green, and blue color filters to create different colors. Since the “white” backlight is theoretically made up of red, green, and blue light blended together, this subtraction method should yield all possible colors.

Yet, as the saying goes, “in theory, there is no difference between theory and practice. But in practice....”

Consider the difference between a typical tungsten light bulb and an overhead office fluorescent lamp, or even one of the newer fluorescent bulbs designed for house lamps. While both are called “white,” most people recognize a difference and describe the tungsten bulb with words like “warmer” or “softer.” That creates a pleasant environment, but the actual “white” leans toward red and orange. Formally speaking, tungsten bulbs have a lower color temperature than white that is made up of equal parts of red, green, and blue light. The Cold Cathode Fluorescent Lamp (CCFL) backlights of traditional LCD monitors similarly diverge from “pure white.” And, when red, green, and blue are subtracted from less than 100 percent white, the result is a mathematically limited range of possible colors.

Several companies, including Brightside, have been experimenting with LED backlights instead of CCFLs and other fluorescent lamps. NEC, for example, now has a 21-inch desktop monitor, the LCD2180, that employs two defused arrays of red, green, and blue LED as the backlight. The combination of red, green, and blue LEDs act like the red, green, and blue light guns of old CRT projectors, blending the primary colors to produce that pure white and, in turn, a wider range of other colors. NEC reports a range of saturated colors that exceeds Adobe RGB color by 9 percent and NTSC color by 4 percent.

Similarly, a development partnership between Samsung and Sony has yielded a small number of products, including Sony’s Qualia 005 (a 46-inch LCD panel) and Samsung’s LN-S8297DE (a gigantic 82-inch LCD panel). Samsung has also been working with Texas Instruments’ DLP (Digital Light Processing) technology to build a DLP rear-projection TV that uses LEDs instead of a traditional projection lamp.

Those products all, like NEC’s, use an array of red, green, and blue LEDs to create a pure, adjustable white backlight, and the visible results in each case are impressive.

Still, the technology ultimately only improves color accuracy and not necessarily contrast.

The Bright Side of LED

To create a true HDR display, Brightside exploits another critical characteristic of LEDs. As solid-state devices, LEDs can be turned on and off extremely quickly-within nanoseconds. They can also shine at reduced power levels to produce a gray light. Most importantly, Brightside modulates the LEDs individual on the fly, adjusting for each display image and even each “field” of 60 field/sec motion video. That fields two important improvements over traditional LCDs.

First, turning the LED backlights off while the liquid crystals are changing state (turning) helps minimize the image ghosting that remains a negative LCD stereotype with video. Second, the DR37-P actually looks ahead a handful of video frames to analyze the picture, then adjusts the LEDs individually to accommodate light and dark scenes and light and dark areas of the scenes. That’s where Brightside really reaches the full potential of high dynamic range.

As I stated earlier, our eyes can distinguish contrast in the 1,000,000:1 range, but that’s not the whole story. At any given instant, we are much more limited-in the order of just 100:1. That’s why you can’t, for example, go to a bright, sunny beach and expect to immediately see something inside a dark tote bag. Our pupils need to dilate and adjust to the new luminance range of the inner tote bag to allow us to discern objects. Similarly, we can readily differentiate objects in dark evening shadows (a survival defense mechanism) because our eyes have adjusted to a different overall luminance.

Brightside’s panel adjusts the brightness levels so that dark and bright environments can be seen clearly.

In order to create a similar range, Brightside uses an array of 1380 LEDs with mildly defused pixels behind the 1920x1080 LCD matrix. Analyzing a given image allows the DR37 to adjust each individual LED, thereby exploiting a full LCD contrast ratio in the various sectors of a single image. Dark areas can have smartly defined shadows and highlights, while still allowing bright areas to shine enough to create the painful glow of looking at the sun.

In that way, Brightside has given an LCD panel something like dilating LEDs that act much like our eyes, adjusting to the brightness and allowing us to see both dark and bright environments clearly. And that, leveraging such a high dynamic range, creates a far more lifelike viewing experience.

Brightside’s technology is still new and quite expensive, but it does offer an attractive glimpse at what is possible for future displays. Minimally, the work of Brightness, NEC, Sony, and Samsung has piqued the interest of LED manufacturers and has yielded some dramatic performance increases during the last couple of years, with more on the horizon. And, that bodes well for the future of HDR displays.

Jeff Sauer is a contributing editor of Computer Graphics World and director of the Digital Video Group, an independent research and testing organization for digital media. He can be reached at jeff@dtvgroup.com.

Computer Graphics World April, 2006
Author(s) :
Jeff Sauer

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